1. Field of the Invention
The present invention relates to a polyimide metal laminate having a silica-dispersed polyimide resin layer excellent in modulus of elasticity at high temperatures, dimensional stability, transparency, and adhesion to an inorganic compound substrate. Specifically, the present invention relates to a polyimide/metal laminate suitable as a substrate for chip on film (hereinafter, COF), which has at least one silica-dispersed polyimide resin layer, hardly permit a chip to sink in a polyimide resin layer upon jointing the chip to a metal wiring via Au—Au jointing or Au—Sn jointing by an inner lead bonder, and is used widely in a tape automated bonding (TAB) tape processing line.
2. Description of the Related Art
A polyimide metal laminate is produced by casting polyimide resin onto metallic foil or laminating a polyimide film with metallic foil. The polyimide/metal laminate can also be produced by sputtering or plating a metal layer on the surface of a polyimide film. The polyimide/metal laminate is widely used as a flexible print substrate and for TAB tape.
In recent years, a wiring pattern for a flexible print substrate obtained from a polyimide film came to be patterned more finely. From the viewpoint of the reliability of a mounding system in TAB line and chip/wiring connection, Au—Au jointing and Au—Sn jointing came to be widely used in chip mounting. However, when chip mounting by Au—Sn jointing is conducted in a polyimide/metal laminate having a thermoplastic polyimide layer formed on metallic foil to improve the adhesion etc. between the metal layer and the polyimide layer, there may arise a problem that wiring and chip bumps sink in the thermoplastic polyimide, the wiring deviates, the wiring is released from the polyimide, and plating soaks. When the wiring considerably sinks, the gap between the chip and the polyimide layer is decreased, thus causing problems such as failure to insert an under film, edge shortening, etc. The wiring deviation causes problems such as short-circuiting due to contact between adjacent wirings. These problems are considered attributable to deformation of the polyimide layer upon chip mounting, and it has been attempted to solve the problems by improving the modulus of elasticity of the polyimide resin layer at high temperatures (chip mounting temperature).
For example, in the method of laminating polyimide on metallic foil by a casting or laminating system, the following two attempts have been made. A first method is a method of producing a polyimide/metal laminate by adding inorganic fillers such as silica and alumina to a polyimide resin layer to improve the modulus of elasticity of the polyimide resin layer. In this method, however, there is a problem that by adding the fillers, the transparency of the polyimide resin layer is lowered so that after circuit processing, an image of metallic wiring cannot be recognized by viewing the face of the polyimide resin layer on which metal is not laminated, thus making positioning upon chip mounting with an inner lead bonder difficult. Another method is a method of producing a polyimide/metal laminate by using highly crystalline polyimide exhibiting high modulus of elasticity. This method is free of the above problem, but has disadvantages such as low production efficiency due to the poor drying properties inherent in the highly crystalline polyimide because such properties cause the air bubbles in the polyimide film upon producing a polyimide/metal laminate.
As a method of producing a metal laminate without using thermoplastic polyimide, a method that involves sputtering metal on a polyimide film is known. A polyimide/metal laminate produced by the sputtering system does not contain a thermoplastic resin layer, and thus there does not occur the phenomenon where a metal wiring sinks in the polyimide layer upon chip mounting. In the sputtering system, however, there is a disadvantage of easy deterioration in yield due to pinholes in the metal layer. To produce a pinhole-free polyimide/metal laminate, lamination of polyimide on metallic foil by a casting or laminating system is effective, but in this case, the above problems occur.
As techniques of improving the adhesion and dimensional stability of polyimide resin, techniques of making a silica/polyimide composite by a sol/gel method are known (see Industrial Materials, Vol. 46, 32 (1998), etc.). Usually, a typical method of finely dispersing silica particles in polyimide includes a method of sol/gel reaction in a solution of polyamic acid as a precursor of polyimide and a method of dehydration imidation of alkoxysilane oligomer-grafted polyamic acid.
As the former method, a method of producing a polyimide composition having silica particles dispersed finely therein, which comprises reacting amino-containing alkoxysilane or a derivative thereof and alkoxysilane in the presence of water in a polyamic acid solution and subsequent polyimidation thereof is disclosed in JP-A 8-73739. However, this patent application merely describes that a film produced by this method is excellent in transparency and tensile modulus of elasticity, and there is no description of specific applications thereof.
As the latter method, a method wherein alkoxy-containing silane-modified polyamic acid produced by reacting polyamic acid with an epoxy-containing alkoxysilane partial condensate is imidated by thermosetting is disclosed in JP-A 2002-293933. In this method, however, a step of introducing a precursor of fine silica particles into a specific position of polyamic acid may be required, and gelation occurs easily due to introduction of an oligomer into the polymer, and thus there is a limit to the type of usable polyimide.
As described above, the conventionally known polyimide/metal laminate has problems such as sinking, deviated wiring, release and plating soaking in the system for jointing of chip via Au—Au jointing, Au—Sn jointing etc. at high temperatures, or problems such as poor transparence of the polyimide layer, low efficiency of production of the laminate, and occurrence of pinholes in metal layer, and cannot cope sufficiently with wiring rendered finer in recent years.